Pack of heat transfer plates

ABSTRACT

A pack of heat transfer plates (2,3) are intended for heat exchangers, preferably for rotary regenerative air preheaters. The plates have mutually parallel S-shaped double ridges (21,22;31,32) and the plates are arranged in the pack (1) so that the double ridges of one plate intersect the double ridges of an adjacent plate and extend symmetrically and obliquely in mutually opposite directions relative to the main flow directions (6) of the heat exchanging media. The throttling effect normally manifest at the double-ridge intersections (24) is avoided by orienting the plates (2,3) so that each pair of double ridges which converge onto an intersection (24) presents a part (21) of the double ridge of the one plate (2) which projects into an intermediate channel, and also a part (32) of the double ridge of the other plate (3) which projects from the intermediate channel.

BACKGROUND OF THE INVENTION

The present invention relates to a pack of heat transfer plates for usein heat exchangers, preferably in rotary regenerative air preheaters.

Heat transfer plates which each have corrugations formed by S-shapeddouble ridges on respective plates and intermediate flat or slightlyundulated plate portions are known, for instance, from British PatentSpecification Nos. 1,335,205 and 1,252,319, respectively. These knownplates have been found to possess optimum values with regard to heattransfer characteristics and low pressure drop conditions. When stackedin wedge-shaped packs in regenerative air preheaters in particular, theadvantages afforded by those known heat transfer plates are utilizedfully in a particularly beneficial manner, since it has been found thatsuch packs are less liable to become blocked by so-called soot and otherparticulate solids present in flue gases than are other types of heattransfer plates used for a similar purpose.

The known packs of plates are produced in accordance with three mutuallydifferent methods. According to the first of these methods, a strip ofsheet metal is first rolled between two profiled rolls and therewithgiven the intended profiled shape, whereafter the thus profiled metalstrip is cut progressively into a large number of pieces, the size ofwhich represents, for instance, the smallest or narrowest dimension of awedge-shaped pack of plates, whereafter the profiled metal strip isagain cut into an equal number of plates, the size of which representsthe next smallest dimension of the pack, and so on until plates havingthe largest desired dimension have been cut from said strip of profiledsheet metal. In this way there is produced a store of plates having thevarious requisite dimensions from which the plates required can becollected in sequence, with successively increasing or decreasingdimensions and the plates stacked upon each other with the double ridgesof mutually adjacent plates intersecting one another, i.e. subsequent toturning each alternate plate through 90°, to form a wedge-shaped pack ofplates. The flow channels defined by mutually contiguous plates willthen have the pattern illustrated in the above mentioned British patentspecifications.

According to the second of these three known methods, respective stripsof sheet metal are rolled simultaneously in separate roll stands whichare mutually so arranged that when the sheet metal strips are fed outfrom respective roll pairs in superposed relationship, the double ridgesof respective strips will intersect one another in the mannerillustrated, for example, in British Patent Specification No. 1,401,621.This method includes cutting both of the sheet metal stripssimultaneously into smaller pieces while successively changing the platedimensions after each cutting operation, so that the pieces cut from thestrips can be stacked immediately in the form of a wedge-shaped body,whereafter the procedure is repeated for the next pack of plates inline.

The third of the aforesaid three known methods can be said to comprise acombination of the first and the second methods. In the case of thethird method, there is used only one roll stand, and subsequent todividing the metal strip into smaller pieces, or plates, the plates areformed into packs, although it is necessary in this case to turn eachalternate plate subsequent to cutting said plate from the strip, so thatthe double ridges on mutually adjacent or contiguous plates willintersect one another.

The sole drawback exhibited by these known plates is that somedifficulty is experienced in effectively blowing away so-called soot andproducts of corrosion, since the soot blowing jets are disintegrated inthe flow channels between the plates by the obliquely extending doubleridges. As a result the channels defined by mutually adjacent orcontiguous plates may become partially blocked sporadically, which maynecessitate shutting down the heat exchange system in order to clean theair preheater.

For this reason this type of heat transfer plate has not been acceptedin some countries, despite being available for seventeen years, while inother countries the plate has been accepted on the merits of its highheat transfer performance and has been used in conjunction withauxiliary solutions for improving the ejection of so-called soot andother solids by blowing. One such solution has involved dividing theheat transfer plates into at least two parts in the direction of mediumflow with an empty space between said two parts, so that so-called sootand other solids can be blown towards the empty space, from both ends ofthe plates. This solution is not an ideal solution, however, either fromthe aspect of blowing the plates clean of soot or from the aspect ofspace.

The object of the present invention is to improve the flow pattern inthis type of plate pack, such as to prevent blocking of the channelsbetween adjacent plates, either partially or completely, in a moreeffective manner.

SUMMARY OF THE INVENTION

This object has been achieved in accordance with the invention bymutually orientating the plates so that each pair of double ridgesconverging towards a point of intersection presents a part of the doubleridges of one plate which protrudes into an intermediate channel, and apart of the double ridges of the other plate which protrudes away fromsaid intermediate channel.

It has been observed that in those instances when blocking has occurred,it has commenced at a location at which a pair of double ridges whichconverge towards an intersection point both present a part of the doubleridges which projects into an intermediate channel. This means that aplate has been turned in a disadvantageous manner, despite the fact thatthe double ridges of the plate intersect the double ridges of mutuallyadjacent plates, and that consequently several intersection points onthis plate have a tendency to initiate blockages. The reason why suchpoints of intersection initiate blockages is because they act in themanner of funnel-shaped constrictions in which particles of so-calledsoot, smuts and other solids, and often sticky particles, collect andimpede the throughflow of heat exchanging media. The jets used to blowaway so-called soot and other solids are also slowed down by theintersections.

In the case of one embodiment according to the invention, allintersections are mutually the same, the essential inventive featurebeing that at each intersection one double ridge will always have a partwhich projects away from the channel and Which Will permit the flow ofmedium to pass the intersection. This arrangement will also enable thejets used to blow away soot etc. to pass through the intersectionwithout necessarily being retarded.

In order to reduce the resistance to flow to the greatest possibleextent, and therewith also the tendency for blockages to occur, theS-shaped double ridges of each plate according to one preferredembodiment are separated in a known manner by a flat portion whose widthis considerably greater than the width between the ridges of each doubleridge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wedge-shaped pack of heat transferplates;

FIG. 2 is a perspective view in partial section which illustrates themedium flow at the ridge intersection point of two superimposed platesconstructed in accordance with the invention;

FIG. 3 is a simplified view of the FIG. 2 illustration taken in thedirection of medium flow; and

FIG. 4 is a view similar to FIG. 3 but illustrating the medium flow inthe case of two plates that are not constructed in accordance with theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates a wedge-shaped pack 1 of heat transfer plates 2, 3which are of the kind illustrated in FIGS. 2 and 3 and which areintended for use in a rotary regenerative air preheater. A preheater ofthis kind will contain a large number of plate packs 1 which form anannular body in the preheater and which present channels for the heattransfer media, these channels extending between two mutually oppositeend surfaces 4, 5 of the respective plate packs 1. The main direction ofmedia flow is indicated by the double arrow 6.

Each plate 2, 3 is provided with corrugations in the form of S-shapeddouble ridges 21, 22 and 31, 32, with a distance 2a between the apicesof respective ridges of each double ridge. The distance between the flatplate portions 23, 33 is thus 2a. As will be seen from FIGS. 2, 3, thedistance between the superimposed plates alternates between 2a and 4a atthe double-ridge intersection points 24, the media flow path at theseintersections being indicated by an arrow 25. This means that particlesof soot, smuts and other solids accompanying the heat transfer media canreadily pass the intersections 24. The conditions are the same in thecase of medium flow from the opposite direction and in adjacent channels(not shown).

For the sake of comparison a study can be made of the events which takeplace in the case of plates which although positioned so that theirdouble ridges mutually intersect are not oriented in relation to oneanother in accordance with claim 1. Such a case is illustrated in FIG.4, in which one plate 40 having double ridges 41, 42 has a differentorientation to the corresponding plate of the FIG. 3 embodiment. In thecase of the FIG. 4 embodiment the portions 21 and 42 of respectivedouble ridges 21, 22 and 41, 42 face one another at the intersections43, such that the distance between the plates is merely a, resulting ina constricted flow passage at the location of said intersections. Thewhole of the channel between the plates 2 and 40 has a throttling effecton the medium flow, particularly when compared with the medium flow inthe two channels (not shown) immediately adjacent the former channel.These two adjacent channels have a larger effective area than the formerchannel and thus conduct a relatively large flow of medium, if alsothese plates should be wrongly oriented. The constrictions occurring atthe intersections 43 are liable to cause particles of soot, smuts andother solids entrained with the medium flow to fasten immediatelyupstream of the intersections 42 and thereafter in the funnel-shapedspaces upstream of said intersections, as seen in the flow direction.Thus, of all the heat transfer plates included in, e.g., the preheater,it suffices that solely one plate is wrongly oriented to initiateclogging or blocking of the flow channels, this initial clogging of thechannels normally spreading rapidly to other flow channels. In order toeliminate all risk of the plates being wrongly oriented, it is necessaryto roll the metal strip and to cut the profiled metal strips into plateform and then to pack the resultant plates automatically in a givensequence, for example with the aid of two roll stands of the kindillustrated in the British Patent Specification No. 1,559,084 withdirect feed to a strip cutting or shearing device and from thereimmediately to a facility for packing the resultant heat transfer platesinto ready-for-use plate packs, in accordance with British PatentSpecification No. 1,401,621. The use of two roll stands obviates theneed to turn each alternate plate prior to gathering the plates intopacks in accordance with hitherto standard procedure, this standardprocedure invariably resulting in the plates of at least a multiple ofplate packs in each air preheater being correctly and incorrectlyoriented in an uncontrollable manner. Thus, both of the British PatentSpecifications mentioned in the introduction illustrate heat transferplates which are so oriented as to oppose the disclosures of the presentclaim 1. When applying the method in which one roll stand is used and inwhich each alternate plate is turned through a given number of degrees,it must be ensured that respective plates are turned always about anaxis at right angles to the main direction of the ultimate channels,i.e. parallel with the directional line along which the profiled stripis fed through the strip cutting device. If this proviso is notfulfilled, the plates will be wrongly oriented. It must also be ensuredthat continuity is maintained subsequent to a breakdown. All of thesepitfalls are avoided when using double roll stands.

We claim:
 1. In a pack of heat transfer plates for heat exchangers,comprising:a plurality of mutually identical profiled plates (2,3) whichare arranged in mutually contiguous relationship and which form channelsfor passage of a heat exchanging media, said channels extending betweentwo mutually opposing end surfaces (4,5) of the pack (1), and in whicheach plate in the pack (1) has provided thereon corrugations in the formof mutually parallel S-shaped double ridges (21,22;31,32) which projectsymmetrically and obliquely from both opposite sides of each said plate;said plates (2,3) being oriented such that the double ridges of oneplate intersect the double ridges of an adjacent plate, and saidadjacent plates being in contact with one another solely at points ofintersection (24) of said intersecting double ridges and; said doubleridges of respective plates extending symmetrically and obliquely inmutually opposite directions relative to main flow directions (6) of theheat exchanging media flowing in the channels formed between adjacentplates; the improvement wherein: said plates (2,3) are mutually orientedsuch that each pair of double ridges which, as viewed in the maindirections (6) of the flows of heat exchanging media, converge towards apoint of intersection (24) presents a part (21) of the double ridge ofone plate (2) which projects into an intermediate channel, and also apart (32) of the double ridge of the other plate (3) which projects awayfrom said intermediate channel, so as to provide substantially equalflow of said heat exchanging media between all plates of said packindependent of the flow direction of the heat exchanging media.
 2. Thepack of heat transfer plates of claim 1, wherein said S-shaped doubleridges of each plate are separated by a flat plate portion (23) whichhas a width that greatly exceeds the width of each double ridge(21,22;31,32).